Method for producing phthalic anhydride

ABSTRACT

A process for preparing phthalic anhydride by gas-phase oxidation of xylene, naphthalene or mixtures thereof over two different fixed-bed catalysts arranged in zones in a shell-and-tube reactor which is thermostated by means of a heat transfer medium is carried out so that the maximum temperature in the second catalyst zone in the flow direction is at least 52° C. lower than the maximum temperature in the first catalyst zone.The process of the present invention makes it possible to prepare phthalic anhydride in high yields under conditions relevant to industrial practice.

The present invention relates to a process for preparing phthalicanhydride by gas-phase oxidation of xylene, naphthalene or mixturesthereof over two different fixed-bed catalysts arranged in zones in ashell-and-tube reactor which is thermostated by means of a heat transfermedium.

It is known that phthalic anhydride is prepared industrially bycatalytic gas-phase oxidation of o-xylene or naphthalene inshell-and-tube reactors. The starting material is a mixture of a gascomprising molecular oxygen, for example air, and the o-xylene and/ornaphthalene to be oxidized. The mixture is passed through a large numberof tubes located in a reactor (shell-and-tube reactor), with each of thetubes containing a bed of at least one catalyst. To regulate thetemperature, the tubes are surrounded by a heat transfer medium, forexample a salt melt. Nevertheless, local temperature maxima (hot spots)in which the temperature is higher than in the remainder of the catalystbed can occur. These hot spots give rise to secondary reactions such astotal combustion of the starting material or lead to the formation ofundesirable by-products which can be separated from the reaction productonly with difficulty, if at all. Furthermore, the catalyst can beirreversibly damaged above a particular hot spot temperature.

The hot spot temperatures are usually in a temperature range from 400 to500° C., in particular from 410 to 460° C. Hot spot temperatures above500° C. lead to a severe decrease in the achievable PA yield and in theoperating life of the catalyst. On the other hand, hot spot temperatureswhich are too low lead to an excessively high content of under-oxidationproducts in the phthalic anhydride (in particular phthalide), resultingin a significant deterioration in the product quality. The hot spottemperature depends on the xylene loading of the air stream, on thespace velocity of the xylene/air mixture over the catalyst, on the stateof aging of the catalyst, on the heat transfer conditions in thefixed-bed reactor (reactor tube, salt bath) and on the salt bathtemperature.

To reduce this hot spot, various measures have been proposed in, forexample, DE 25 46 268 A, EP 286 448 A, DE 29 48 163 A, EP 163 231 A, DE41 09 387 A, WO 98/37967 and DE 198 23 362 A. In particular, asdescribed in DE 40 13 051 A, a change has been made to arrangingcatalysts of differing activity in zones in the catalyst bed, with theless active catalyst generally being located closer to the gas inlet andthe more active catalyst being located closer to the gas outlet. Theprocess is carried out using a two-stage salt bath, with the salt bathtemperature of the first reaction zone in the flow direction of thereaction mixture being kept 2-20° C. higher than the salt bathtemperature of the second reaction zone. The catalyst volume in thefirst reaction zone is from 30 to 75% by volume and that in the secondreaction zone is from 25 to 70% by volume. The temperature of the hotspot in the first reaction zone is higher than that in the secondreaction zone. The difference between the hot spot temperatures in themodes of operation described in the examples is considerably less than50° C.

DE 28 30 765 A describes a shell-and-tube reactor in which a catalyst ispresent in two reaction zones and which is suitable for, inter alia, thepreparation of phthalic anhydride. The reaction temperature in thesecond reaction zone from the gas inlet is higher than that in the firstreaction zone.

DE 29 48 163 A describes a process for preparing phthalic anhydrideusing two different catalysts arranged in zones, with the catalyst ofthe first zone making up from 30 to 70% of the total length of thecatalyst bed and the catalyst of the second zone making up from 70 to30% of the total length of the catalyst bed. This is said to reduce thetemperature of the hot spot. However, it has been found that the yieldof phthalic anhydride even at the low o-xylene loadings in the startinggas mixture (maximum 85 g/standard m³) employed in this publicationleaves something to be desired. A similar process is disclosed in DE 3045 624 A.

DE 198 23 262 describes a process for preparing phthalic anhydride usingat least three coated catalysts arranged one above the other in zones,with the catalyst activity increasing from zone to zone from the gasinlet end to the gas outlet end. In this process, the difference in thehot spot temperature from catalyst to catalyst is not more than 10° C.

It is an object of the present invention to provide a process forpreparing phthalic anhydride which gives high yields of phthalicanhydride even at high o-xylene or naphthalene loadings and at highspace velocities.

We have found that this object is achieved by carrying out thepreparation of phthalic anhydride over two catalysts having differingactivities and arranged in zones and controlling the process in such away that the hot spot temperature in the catalyst zone furthest from thegas inlet (in the flow direction) is at least 52° C. lower than that inthe catalyst zone closest to the gas inlet.

The present invention accordingly provides a process for preparingphthalic anhydride by gas-phase oxidation of xylene, naphthalene ormixtures thereof over two different fixed-bed catalysts arranged inzones in a shell-and-tube reactor which is thermostated by means of aheat transfer medium, wherein the maximum temperature (hot spottemperature, i.e. localized region of highest temperature in a catalystzone) in the second catalyst zone furthest from the gas inlet is atleast 52° C. lower than the maximum temperature in the first catalystzone.

The maximum temperature in the second catalyst zone is preferably atleast 55° C., in particular at least 60° C., lower than the maximumtemperature in the first catalyst zone. However, the reaction isgenerally controlled so that the maximum temperature in the firstcatalyst zone is not more than 75° C., in particular not more than 70°C., higher than that in the second catalyst zone. The temperaturedifference is thus preferably in the range from 52 to 75° C.

Furthermore, the process is carried out so that the hot spot temperaturein the first catalyst zone is less than 470° C., preferably less than450° C.

The difference in the hot spot temperatures can be adjusted in variousways. For example, it can be done by increasing the admission pressureof the starting gas mixture by up to 10% or by lowering the amount ofair used for the oxidation by up to 20%. However, the temperaturedifference is preferably controlled by means of the bed length ratio ofthe two catalysts or by means of the temperature of the heat transfermedium (hereinafter, reference will always be made to the preferred heattransfer medium, namely a salt bath), in particular when the twocatalyst zones are thermostated by means of different salt bathcircuits. The bed length of the first catalyst zone preferably makes upmore than 60%, in particular more than 70% and particularly preferablymore than 75%, of the total height of the catalyst bed in the reactortube.

If the salt bath temperature is used for control, an increase in thesalt bath temperature leads to an increase in the hot spot temperaturein the first catalyst zone and to a decrease in the second catalystzone. For this reason, a slight increase or decrease, e.g. by 1, 2 or 3°C., is generally sufficient to set the desired hot spot temperaturedifference. If the two catalyst zones are thermostated by means ofdifferent salt bath circuits, the upper salt bath circuit, i.e. the saltbath circuit which thermostats the first catalyst zone, is preferablyoperated at a temperature which is 1-5° C. higher than that of the lowersalt bath circuit. Alternatively, the temperature of the salt bath whichthermostats the second catalyst zone is decreased by up to 20° C.

The operating life of the catalyst is generally from about 4 to 5 years.The activity of the catalyst generally decreases somewhat over thecourse of time. As a result, the hot spot temperature difference candrop below the minimum value of 52° C. It can then be restored to avalue above 52° C. by increasing the salt bath temperature as describedabove. The process is preferably carried out so that the hot spottemperature difference is maintained for at least the first 50%, inparticular at least the first 70%, particularly preferably at least thefirst 90%, of the operating life of the catalyst and particularlyadvantageously during essentially the entire operating life of thecatalyst.

The hot spot temperature is determined in a known manner, e.g. byinstallation of a plurality of thermocouples in the reactor.

Supported oxidic catalysts are suitable as catalysts. In the preparationof phthalic anhydride by gas-phase oxidation of o-xylene or naphthalene,use is made of spherical, ring-shaped or dish-shaped supports comprisinga silicate, silicon carbide, porcelain, aluminum oxide, magnesium oxide,tin dioxide, rutile, aluminum silicate, magnesium silicate (steatite),zirconium silicate or cerium silicate or mixtures thereof. Catalyticallyactive constituents are generally titanium dioxide, particularly in theform of its anatase modification, together with vanadium pentoxide. Inaddition, the catalytically active composition may further comprisesmall amounts of many other oxidic compounds which act as promoters toinfluence the activity and selectivity of the catalyst, for example byreducing or increasing its activity. Examples of such promoters arealkali metal oxides, thallium(I) oxide, aluminum oxide, zirconium oxide,iron oxide, nickel oxide, cobalt oxide, manganese oxide, tin oxide,silver oxide, copper oxide, chromium oxide, molybdenum oxide, tungstenoxide, iridium oxide, tantalum oxide, niobium oxide, arsenic oxide,antimony oxide, cerium oxide and phosphorus pentoxide. The alkali metaloxides act, for example, as promoters which reduce the activity andincrease the selectivity, while oxidic phosphorus compounds, inparticular phosphorus pentoxide, increase the activity of the catalystbut reduce its selectivity. Catalysts which can be used are described,for example, in DE 25 10 994, DE 25 47 624, DE 29 14 683, DE 25 46 267,DE 40 13 051, WO 98/37965 and WO 98/37967. Coated catalysts in which thecatalytically active composition is applied in the form of a shell tothe support (cf., for example, DE 16 42 938 A, DE 17 69 998 A and WO98/37967) have been found to be particularly useful.

The less active catalyst is arranged in the fixed bed so that thereaction gas comes into contact with this catalyst first and only thencomes into contact with the more active catalyst in the second zone. Thecatalysts having differing activities can be thermostated to the sametemperature or to different temperatures. In general, a catalyst dopedwith alkali metal oxides is used in the first catalyst zone closest tothe gas inlet and a catalyst doped with smaller amounts of alkali metaloxides and/or phosphorus compounds and/or further promoters is used inthe second reaction zone.

Particular preference is given to catalysts having the followingcomposition:

for the first zone:

from 3 to 5% by weight of vanadium pentoxide

from 0.1 to 1% by weight of an alkali metal oxide, e.g. cesium oxide

from 94 to 96.9% by weight of titanium dioxide

for the second zone:

from 6 to 9% by weight of vanadium pentoxide

from 0 to 0.3% by weight of an alkali metal oxide, e.g. cesium oxide

from 0.05 to 0.4% by weight of phosphorus pentoxide (calculated as P)

if desired, from 1 to 5% by weight of a further promoter, in particularSb₂O₃

from 85.3 to 93.95% by weight of titanium dioxide

In general, the reaction is carried out in such a way that the majorpart of the o-xylene and/or naphthalene present in the reaction gas isreacted in the first reaction zone.

For the reaction, the catalysts are introduced into the tubes of ashell-and-tube reactor so as to form adjacent zones. The reaction gas ispassed over the catalyst bed prepared in this way at temperatures ofgenerally from 300 to 450° C., preferably from 320 to 420° C. andparticularly preferably from 340 to 400° C., and a gauge pressure ofgenerally from 0.1 to 2.5 bar, preferably from 0.3 to 1.5 bar, and at aspace velocity of generally from 750 to 5000 h⁻¹, preferably from 2000to 5000 h⁻¹. The reaction gas (starting gas mixture) fed to the catalystis generally produced by mixing a gas which comprises molecular oxygenand may further comprise suitable reaction moderators and/or diluentssuch as steam, carbon dioxide and/or nitrogen, with the o-xylene ornaphthalene to be oxidized. The reaction gas generally contains from 1to 100 mol %, preferably from 2 to 50 mol % and particularly preferablyfrom 10 to 30 mol %, of oxygen. In general, the reaction gas is ladenwith from 5 to 140 g of o-xylene and/or naphthalene per standard m³ ofgas, preferably from 60 to 120 g of o-xylene and/or naphthalene perstandard m³ of gas and particularly preferably from 80 to 120 g ofo-xylene and/or naphthalene per standard m³ of gas.

If desired, a downstream finishing reactor, as described, for example,in DE 198 07 018 or DE 20 05 969 A, can be additionally provided for thepreparation of phthalic anhydride. As catalyst for this finishingreactor, preference is given to using a catalyst which is even moreactive than the catalyst of the second zone. In particular, thiscatalyst has the following composition:

from 6 to 9% by weight of vanadium pentoxide

from 1 to 5% by weight of an activity-increasing promoter, in particularSb₂O₃

from 0.1 to 0.5% by weight of phosphorus pentoxide (calculated as P)

from 85.5 to 92.9% by weight of titanium dioxide.

The process of the present invention has the advantage that phthalicanhydride can be prepared in high yield and with low concentrations ofby-products, in particular phthalide, even at high loadings of o-xyleneand/or naphthalene and at high space velocities. Under the conditions ofthe process of the present invention, the phthalide concentration is nohigher than 0.1% by weight, based on PA. The advantages of the processof the present invention are particularly evident when the activity ofthe catalyst system used decreases due to aging. Even after a longperiod of operation, there is only an insignificant increase in the hotspot in the second catalyst zone.

The temperature control used according to the present invention can alsobe employed in the preparation of other products by catalytic gas-phaseoxidation, e.g. acrylic acid (from propene), maleic anhydride (frombenzene, butene or butadiene), pyromellitic anhydride (from durene),benzoic acid (from toluene), isophthalic acid (from m-xylene),terephthalic acid (from p-xylene), acrolein (from propene), methacrylicacid (from isobutene), naphthoquinone (from naphthalene), anthraquinone(from anthracene), acrylonitrile (from propene) and methacrylonitrile(from isobutene).

The following examples illustrate the invention without restricting itsscope.

EXAMPLES

1) Production of the Catalysts I-III

Catalyst I:

50 kg of steatite (magnesium silicate) rings having an external diameterof 8 mm, a length of 6 mm and a wall thickness of 1.5 mm were heated to160° C. in a coating drum and sprayed with a suspension comprising 28.6kg of anatase having a BET surface area of 20 m²/g, 2.19 kg of vanadyloxalate, 0.176 kg of cesium sulfate, 44.1 kg of water and 9.14 kg offormamide until the weight of the applied layer was 10.5% of the totalweight of the finished catalyst (after calcination at 450° C.).

The catalytically active composition applied in this way, i.e. thecatalyst shell, comprised 4.0% by weight of vanadium (calculated asV₂O₅), 0.4% by weight of cesium (calculated as Cs) and 95.6% by weightof titanium dioxide.

Catalyst II

50 kg of steatite (magnesium silicate) rings having an external diameterof 8 mm, a length of 6 mm and a wall thickness of 1.5 mm were heated to160° C. in a coating drum and sprayed with a suspension comprising 28.6kg of anatase having a BET surface area of 20 m²/g, 4.11 kg of vanadyloxalate, 1.03 kg of antimony trioxide, 0.179 kg of ammonium dihydrogenphosphate, 0.045 kg of cesium sulfate, 44.1 kg of water and 9.14 kg offormamide until the weight of the applied layer was 10.5% of the totalweight of the finished catalyst (after calcination at 450° C.).

The catalytically active composition applied in this way, i.e. thecatalyst shell, comprised 0.15% by weight of phosphorus (calculated asP), 7.5% by weight of vanadium (calculated as V₂O₅), 3.2% by weight ofantimony (calculated as Sb₂O₃), 0.1% by weight of cesium (calculated asCs) and 89.05% by weight of titanium dioxide.

Catalyst III

50 kg of steatite (magnesium silicate) rings having an external diameterof 8 mm, a length of 6 mm and a wall thickness of 1.5 mm were heated to160° C. in a coating drum and sprayed with a suspension comprising 28.6kg of anatase having a BET surface area of 11 m²/g, 3.84 kg of vanadyloxalate, 0.80 kg of antimony trioxide, 0.239 kg of ammonium dihydrogenphosphate, 44.1 kg of water and 9.14 kg of formamide until the weight ofthe applied layer was 12.5% of the total weight of the finished catalyst(after calcination at 450° C.).

The catalytically active composition applied in this way, i.e. thecatalyst shell, comprised 0.2% by weight of phosphorus (calculated asP), 7.0% by weight of vanadium (calculated as V₂O₅), 2.5% by weight ofantimony (calculated as Sb₂O₃) and 90.3% by weight of titanium dioxide.

2) Oxidation of O-Xylene

2a) Preparation of PA—According to the Present Invention and Comparison

(Setting of the Hot Spot Temperature Difference by Variation of BedLength)

In a 10 l tube reactor (99 normal tubes and 2 thermocouple tubes),firstly (at the bottom) catalyst II (comparison: 1.30 m; according tothe present invention: 0.70 m) and subsequently catalyst I (comparison:1.70 m; according to the present invention: 2.30 m) were in each caseplaced in each of the 3.60 m long iron tubes having an internal diameterof 25 mm (thermocouple tubes 29 mm with thermocouple sheath of 10 mminternal diameter and 30 installed thermocouples (every 10 cm)). Bymeans of pressure balancing, it was ensured that the same inlet pressureprevailed at each tube inlet. If necessary, a little catalyst I wasadded or removed to/from the 99 normal tubes; in the case of the 2thermocouple tubes, pressure balancing was achieved by addition of inertmaterial in the form of steatite spheres or quartz spheres. To regulatethe temperature of the iron tubes, they were surrounded by a salt meltwhich was located in two separate salt baths. The lower salt bathsurrounded the tubes from the lower tube plate to a height of 1.30 m,and the upper salt bath surrounded the tubes from the height of 1.30 mto the upper tube plate. 4.0 standard m³/h per tube of air laden with100 g of 98.5% strength by weight of o-xylene per standard m³ of airwere passed through the tubes from the bottom upward (after a running-uptime of about two months). After leaving the main reactor, the crudeproduct gas stream was cooled to 280-290° C. and passed through anadiabatic finishing reactor (internal diameter: 0.45 m, height: 0.99 m)charged with 100 kg of catalyst III.

The data listed in the following table were obtained in theseexperiments (day=day of operation starting from the first start-up ofthe catalyst; SBT top=salt bath temperature of the salt bath nearest thereactor inlet; SBT bottom=salt bath temperature of the salt bath nearestthe reactor outlet; HS top=hot spot temperature of the catalyst nearestthe reactor inlet; HS bottom=hot spot temperature of the catalystnearest the reactor outlet; PHDE or xylene content=phthalide or xylenecontent of the crude product gas after the finishing reactor, based onphthalic anhydride; PA yield=PA yield in % by weight based on 100%-purexylene from the analysis of the crude product gas after the finishingreactor.

Tem- pera- SBT ture top/ dif- SBT HS HS fer- PA Day bottom top bottomence yield Bed [d] [° C.] [° C.] [° C.] [° C.] [m/m %] Com- 100 348/348434 366 68 113.1 parison 150 348/348 434 375 57 112.9 170/130 200348/348 421 390 31 112.0 250 348/348 419 394 25 111.3 According 100348/348 430 362 68 113.3 to the 150 348/348 431 363 68 113.1 present 200348/348 425 368 57 112.9 invention 250 348/348 421 371 50 112.7 230/70

2b) Preparation of PA—According to the Present Invention (TemperatureVariation and Temperature Structuring)

For the catalyst combination operated as comparative experiment in 2a),a temperature difference of >40° C. was set after operation for 250 daysby means of temperature structuring (SBT bottom decreased or SBT topincreased) or temperature variation (SBT bottom and top increased). Allother experimental conditions ere not changed from those in Experiment2a).

The data listed in the following table were obtained in this experiment(day=day of operation starting from the first start-up of the catalyst;SBT top=salt bath temperature of the salt bath nearest the reactorinlet; SBT bottom=salt bath temperature of the salt bath nearest thereactor outlet; HS top=hot spot temperature of the catalyst nearest thereactor inlet; HS bottom=hot spot temperature of the catalyst nearestthe reactor outlet; PHDE or xylene content=phthalide or xylene contentof the crude product gas after the finishing reactor, based on phthalicanhydride; PA yield=PA yield in % by weight based on 100%-pure xylenefrom the analysis of the crude product gas after the finishing reactor.

Tem- pera- SBT ture top/ dif- SBT HS HS fer- PA Bed Day bottom topbottom ence yield 170/130 [d] [° C.] [° C.] [° C.] [° C.] [m/m %]Comparison without 250 348/348 419 394 25 111.3 temperature structuringAccording to the 252 349/349 428 387 41 112.3 present invention 254350/350 437 381 56 112.5 with temperature increase According to the 256349/348 429 385 44 112.5 present invention 258 350/348 438 379 58 112.8with temperature 260 348/343 419 381 38 112.0 structuring 262 348/338418 370 48 112.9 264 348/335 419 365 54 113.1

The results reported under 2a) show that the PA yield correlates withthe hot spot temperature difference, i.e. under operating conditionsrelevant to practice, PA is obtained in a higher yield when thetemperature difference is above 52° C.

The results reported under 2b) show that when the hot spot temperaturedifference is too small, increasing the salt bath temperature top andbottom simultaneously and slightly or reducing the temperature of thelower salt bath by keeping the temperature of the upper salt bathconstant is sufficient to increase the hot spot temperature differenceto more than 52° C.

We claim:
 1. A process for preparing phthalic anhydride by gas-phaseoxidation of xylene, naphthalene or mixtures thereof over two differentfixed-bed catalysts arranged in zones in a shell-and-tube reactor whichis thermostated by means of a heat transfer medium, wherein the maximumtemperature in the second catalyst zone in the flow direction is atleast 52° C. lower than the maximum temperature in the first catalystzone.
 2. A process as claimed in claim 1, wherein the maximumtemperature in the second catalyst zone is at least 55° C. lower thanthat in the first catalyst zone.
 3. A process as claimed in claim 1,wherein the maximum temperature in the second catalyst zone is at least60° C. lower than that in the first catalyst zone.
 4. A process asclaimed in claim 1, wherein the temperature difference between themaximum temperature in the first and second catalyst zones is controlledvia the bed length ratio of the catalyst zones.
 5. A process as claimedin claim 4, wherein the bed length of the first catalyst zone is morethan 60% of the total bed length of the two catalysts.
 6. A process asclaimed in claim 4, wherein the bed length of the first catalyst zone ismore than 75% of the total bed length of the two catalysts.
 7. A processas claimed in claim 1, wherein the temperature difference between themaximum temperature in the first and second catalyst zones is controlledvia the temperature of the heat transfer medium.
 8. A process as claimedin claim 1, wherein the maximum temperature in the first catalyst zoneis less than 470° C.
 9. A process as claimed in claim 1, wherein a gasphase having a loading of from 80 to 140 g of o-xylene and/ornaphthalene per standard m³ of gas phase is used.
 10. A process asclaimed in claim 1, wherein the temperature of the heat transfer mediumis ≦360° C.
 11. A process as claimed in claim 1, wherein the spacevelocity of the gas mixture is ≧2000 h⁻¹.